1. Field of the Invention
The present invention relates to an optical disk on which data is recorded in a high density manner. More specifically, this invention relates to a high density optical disk on which pits smaller than those of a conventional compact disk (referred to as a CD, simply) can be recorded and further data can be recorded and/or reproduced by a laser beam having a wavelength band shorter than the wavelength band (near infrared ray band) of a laser beam source used for a conventional CD player.
2. Description of the Prior Art
Recently, there has been studied a method of increasing the recording density of the optical disk by use of a laser beam source having wavelengths shorter than those of the near infrared ray band (770 nm to 830 nm) so far used in practice for a so-called compact disk system (referred to as CD system, hereinafter). This is because a practical use of a system using a semiconductor laser beam having wavelengths on the order of 600 nm or a laser beam using SHG (Second Harmonic Generation) having wavelengths on the order of 400 to 500 nm, for instance has become close at hand. When the wavelengths of the reproducing laser beam are shortened as described above, since the diameter of the spots of the reproducing laser beam can be reduced, it has become possible to shorten the shortest pit length and further to narrow the track pitch in comparison with the conventional CD. It has also become possible to increase the recording density per unit area, and thereby, a quantity of data larger than that of the conventional CD can be recorded in an optical disk whose diameter is the same as that of the conventional CD (e.g., CD of 12 cm in diameter).
However, it is impossible to reproduce data recorded on the above-mentioned high density recording type optical disk (referred to as high density optical disk, hereinafter) on which pits are recorded in a high density in correspondence to the laser beam of shorter wavelength band, when the conventional CD player is used. When the high density optical disk is mounted on the conventional CD player erroneously, it has been confirmed that the following problems arise:
When a conventional optical disk (CD) is mounted on the conventional CD player, the conventional CD player generally executes an automatic initial operation in accordance with the following sequence:
______________________________________ (1) focus servo ON (2) focus lock OK (3) turntable ON (4) tracking servo ON ______________________________________
In other words, whenever the high density optical disk is mounted on the conventional CD player of the above-mentioned sequential operation, the conventional CD player executes the above-mentioned initial operation.
Here, in the case where the light reflection factor (reflectivity) of the mounted high density disk is of the same extent of that of the CD, there arises no problem until the mode of Item (2) above.
With respect to the turntable ON of Item (3) above, there are certain methods designed into the CD players. For instance, there exists such a typical method wherein a T.T. (turntable) motor is set to a first control mode and the turntable is rotated in a speed control mode wherein the maximum frequency 3T of the reproduced signals is set to the vicinity of a value (i.e., 696 ns) prescribed for the CD. In this method, however, when the mounted disk is of high density type, since the conventional CD player cannot read pits recorded on the mounted high density optical disk, the speed control mode will not function.
Further, a time difference is provided between the mode of Item (3) and the mode of Item (4) according to the design of the CD player. For instance, there are CD players that provide a time difference of about 1 to 3 sec between the modes of Items (3) and (4).
Further, with respect to the mode of Item (4), when a linear speed is low, since the frequency components of the reproduced signals are relatively low and thereby the tracking detection signal is obstructed by the reproduced signals, it is necessary to set the linear speed of the optical disk to such a prescribed value that the tracking servo can be stabilized. In this case, however, when the speed control mode does not function as described above, the disk rotates at a speed higher than 600 r.p.m. Where an unbalance force generated by the optical disk rotating at a relatively high speed exceeds a clamping force, there arise various problems such as vibration, contact between the disk and the player, scratches of the disk and the player, etc.
Further, there exist some players in which the function of detecting the disk rotation stop is not sufficient. In the case of such players, some tests have confirmed that a tray is shifted outside as in an unload mode in spite of the fact that the disk is rotating at a high speed (i.e., not yet stopped). In this case, the disk and the player are both scratched.